1. Field of the Invention
The present invention is concerned with an implantable measuring device, particularly a measuring device for determining the intacardial or intraluminal blood pressure.
2. Background Art
Regarding the background of the invention it can be stated, based on the example of an implantable pressure measuring device, that measuring the intracardial or intraluminalxe2x80x94e.g., arterialxe2x80x94blood pressure is of importance in pacemaker therapy. The average arterial blood pressure for example, is a physiological parameter with the aid of which the stimulation rate of a rate adaptive cardiac pacemaker is adaptable. The intracardial pressure curve, as a further example for the application of pressure measurements, can provide information regarding the activity of the heart valves, the contraction sequence of the heart, and its contractility. In optimal circumstances physiological parameters of this type will be monitored permanently, as they may be continually needed as input parameters for a whole variety of applications.
A permanent operation of implantable pressure measuring devices of this type, such as electromechanical pressure measuring catheters, for example, is not practically feasible at this time as the sensor surfaces will gradually become covered due to the clot formation coagulation and endothelialization occurring on the same because of their constant actuation by the blood flow. This causes their sensitivity to be significantly reduced. This also holds true, for example, for optical sensors that are able to determine the blood oxygen saturation by measuring the amplitude ratio of two peaks in the infrared radiation band, the measuring properties of which ate also significantly deteriorated by the above-mentioned covering processes. For the above reasons implantable measuring sensors and especially pressure sensors are thus used only temporarily.
Measures to eliminate impurities caused by the blood flow arc already known from the prior art in the general context of intracorporally implanted catheters. U.S. Pat. No. 4,698,058 for example, reveals a perfusion catheter, the fluid-carrying lumen of which is cleaned by means of ultra-sound. The sound waves are transmitted via an additional lumen that houses the inner catheter tube with the lumen that is to be cleaned. However, perfusion catheters are generally implanted only temporarily. The impurities that are deposited in its lumen are primarily due to the various chemical or blood-containing fluids that are routed through the catheter in both directions.
U.S. Pat. No. 4,509,947 reveals the cleaning of an implanted medication infusion pump by means of ultrasound. However, this method does not serve to remove deposits caused by the contact of the infusion pump wit body fluids, but rather to remove crystallization that might occur from the medication reservoir.
The ultrasound cleaning devices according to the above printed publications obviously do not pertain to the problems of blood-generated impurities collecting on permanently implanted catheters. These printed publications, therefore, merely constitute technological background.
The present invention is now based on the object of equipping an implantable measuring device in such a way that it can be permanently positioned inside a body cavity that transports or contains a body medium that results in removal of deposits on the measuring device sensor.
This object is met with by the invention, which provides for a cleaning element that is coupled to the sensor is provided in the form of a piezoelectric, preferably piezoceramic actuator, which can be set in vibrations by electrical excitation via supply lines. These vibrations are conveyed to the sensor, setting the same in mechanical oscillations and thus removing the deposits. This cleaning process may be performed in regular intervals or as needed. The frequency and amplitude of the vibration signal may similarly also either be predetermined or opted automatically based on the degree of impurities or effect of the cleaning. The degree of impurities or effect of the cleaning may be determined based on the electrical, optical or mechanical properties of the measuring sensor.
Thin, block-shaped or discoid plate elements composed of piezoceramics, the large faces of which are at least partially covered with electrically conducting contact areas for connecting the supply lines, have proven to be the preferred embodiment for the actuator of the cleaning element. An actuator of this type then preferably lies flat against the sensor that is to be cleaned so that the most optimal transfer of the cleaning vibrations can take place.
Since primarily piezoceramic elements are used as the pressure sensitive sensors in pressure measuring catheters per se, a particularly preferred embodiment of the invention provides for a pressure sensitive sensor and actuator to be formed jointly by a uniform piezoceramic plate element. The signal line and supply line may then correspondingly also be formed by a single common pair of leads that is connected both to the measuring signal evaluation unit for the pressure measurement as well as to a control circuit for actuating the vibration of the plate element.
According to preferred embodiments of the invention, in order to optimize the cleaning effect, provision is made for th: vibration frequency of the actuator to be matched to its self-resonant frequency in its longitudinal or thickness direction. The actuator may Evermore be provided with micro-channels for generating micro flows of gas bubbles or fluid. These micro-channels, which act like nobles, operate according to the same method of operation as the print head of an ink jet printer and thus do not require any detailed explanation.
Lastly, for an optimized energy supply to the implantable measuring device with respect to a ideally unlimited service life, the inductive input of the electrical energy required to generate the actuator vibrations may be provided via an antenna. The latter may be located in the implant in its so-called header, in the supply line to the sensor, or on the sensor itself.
Further characteristics, details and advantages of the invention will become apparent from the following description in which an exemplary embodiment will be explained in more detail based on the appended drawings.